Epidemiological and experimental studies have implicated dietary fat in the development of mammary tumors. To assess the possible direct effects of lipids on mammary cells, the affects of lipids on normal and neoplastic rat mammary cell growth in culture have been analyzed. Saturated fatty acids were found to inhibit while unsaturated fatty acids stimulated the growth of cells from either source. These results may be physiologically important since there is an enrichment of total unsaturated fatty acids in the glands of animals given periphenazine to stimulate proliferation. Additionally, mammary tissue has been shown to differ in the amount and types of free fatty acids released in explant culture depending on whether it contains the epithelial component or not. Less unsaturated fatty acids are released into the medium relative to saturated fatty acids when the epithelium is present in the explanted tissue. These results suggest that normal mammary epithelium recruits unsaturated fatty acids needed for growth from the surrounding adipose tissue. If this process is rate limiting for mammary cell growth then increased dietary lipids may directly supply the fatty acids to the epithelium and bypass this rate limiting process.